Powder coatings

ABSTRACT

The invention relates to powder coatings which are characterized in that they contain as film-forming material a mixture of 
     (A) from 19.9 to 90.0 percent by weight of a synthetic resin which contains epoxide groups and may contain ethylenically unsaturated double bonds, or of a mixture of such epoxide group-containing synthetic resins, 
     (B) from 9.9 to 80.0 percent by weight of a compound which contains carboxyl groups and may contain ethylenically unsaturated double bonds, or of a mixture of such carboxyl group-containing compounds, 
     (C) from 0 to 20 percent by weight of a compound which contains ethylenically unsaturated double bonds and is different from (A) and (B), or of a mixture of such compounds, and 
     (D) from 0.1 to 3.0 percent by weight of an initiator for free-radical polymerizations or of a mixture of initiators for free-radical polymerizations, 
     the percentages by weight of (A)+(B)+(C)+(D) being=200% by weight and the mixture of components (A), (B), (C) and (D) containing from 0.1 to 6.0 mol of ethylenically unsaturated double bonds per 1000 g of mixture.

The invention relates to powder coatings which provide coating filmshaving excellent properties, and in particular coating films of highflexibility with a high degree of hardness.

Powder coatings are known. Since they contain no solvents, they can beused for coating in an especially environment-friendly manner. Thecoating films which can be produced using powder coatings, however,remain in need of further improvement in terms of their properties.

The object on which the present invention is based consists in theprovision of new powder coatings which can be used to produce coatingfilms which, in relation to the prior art, have improved properties, andin particular a higher flexibility and a higher degree of hardness.

This object is achieved, surprisingly, by the provision of powdercoatings which contain as film-forming material a mixture of

(A) from 19.9 to 90.0 percent by weight of a synthetic resin whichcontains epoxide groups and may contain ethylenically unsaturated doublebonds, or of a mixture of such epoxide group-containing syntheticresins,

(B) from 9.9 to 80.0 percent by weight of a compound which containscarboxyl groups and may contain ethylenically unsaturated double bonds,or of a mixture of such carboxyl group-containing compounds,

(C) from 0 to 20 percent by weight of a compound which containsethylenically unsaturated double bonds and is different from (A) and(B), or of a mixture of such compounds, and

(D) from 0.1 to 3.0 percent by weight of an initiator for free-radicalpolymerizations or of a mixture of initiators for free-radicalpolymerizations,

the percentages by weight of (A)+(B)+(C)+(D) being=100% by weight andthe mixture of components (A), (B), (C) and (D) containing from 0.1 to6.0 mol of ethylenically unsaturated double bonds per 1000 g of mixture.

Using the powder coatings provided in accordance with the invention ithas, surprisingly, been possible to produce coating films which, inrelation to the prior art, have improved properties, and in particularhigher flexibilities and higher degrees of hardness.

EP-A-38 635 describes powder coatings which contain, as film-formingmaterial, a synthetic resin containing epoxide groups and a linearpolyester containing carboxyl groups. Coating films prepared using thesepowder coatings possess, in comparison to coating films which have beenproduced using the powder coatings provided in accordance with theinvention, poorer properties, and in particular poorer flexibilities anddegrees of hardness.

The film-forming material of the powder coatings according to theinvention comprises from 19.9 to 90.0 percent, preferably from 39.9 to80.0 percent and particularly preferably from 49.9 to 70.0 percent byweight of component (A), from 9.9 to 80.0 percent, preferably from 19.9to 60.0 percent and particularly preferably from 29.9 to 50.0 percent byweight of component (B), from 0 to 20 percent, preferably from 0.5 to10.0 percent and particularly preferably from 1.0 to 5.0 percent byweight of component (C) and from 0.1 to 3.0 percent, preferably from 0.2to 2.0 percent and particularly preferably from 0.5 to 1.5 percent byweight of component (D), the percentages by weight being based on(A)+(B)+(C)+(D)=100% by weight and the mixture of components (A), (B),(C) and (D) containing from 0.1 to 6.0 mol, preferably from 0.2 to 4.0mol and particularly preferably from 0.3 to 2.5 mol of ethylenicallyunsaturated double bonds per 1000 g of mixture.

As component (A) it is possible in principle to employ any syntheticresin which contains epoxide groups and may contain ethylenicallyunsaturated double bonds, which can be employed for the production ofpowder coatings and has a melting point of from 20° to 100° C.,preferably from 30° to 80° C. and particularly preferably from 30° to60° C., or a mixture of such synthetic resins. Examples of epoxidegroup-containing synthetic resins which may be employed are epoxidegroup-containing polyacrylate resins, polyglycidyl ethers of aliphaticor cycloaliphatic alcohols such as ethylene glycol, diethylene glycol,1,2-propylene glycol, 1,4-butylglycol, 1,2-cyclohexanediol, 1,4cyclohexanediol, 1,2-bis(hydroxymethyl)cyclohexane and hydrogenatedbisphenol A, or polyglycidyl ethers of polyphenols such as bisphenol A,1,1-bis(4-hydroxyphenyl)ethane and2-methyl-1,1-bis(4-hydroxyphenyl)propane, and the compounds containingepoxide groups which are listed in U.S. Pat. No. 4,102,942 in column 3,lines 1 to 16.

As component A) it is preferred to employ an epoxide group-containingpolyacrylate resin or a mixture of epoxide group-containing polyacrylateresins.

The term epoxide group-containing polyacrylate resin refers to a polymerwhich can be prepared by copolymerization of at least one ethylenicallyunsaturated monomer, containing at least one epoxide group in themolecule, with at least one further ethylenically unsaturated monomerwhich contains no epoxide group in the molecule, at least one of themonomers being an ester of acrylic acid or methacrylic [sic] acid.

Epoxide group-containing polyacrylate resins are known (cf. e.g.EP-A-299 420, DE-B-22 14 650, U.S. Pat. No. 4,091,048 and U.S. Pat. No.3,781,379).

Examples of ethylenically unsaturated monomers containing at least oneepoxide group in the molecule are glycidyl acrylate, glycidylmethacrylate and allyl glycidyl ether.

Examples of ethylenically unsaturated monomers containing no epoxidegroup in the molecule are alkyl esters of acrylic and methacrylic acidwhich contain 1 to 20 carbon atoms in the alkyl radical, and inparticular methyl acrylate, methyl methacrylate, ethyl acrylate, ethylmethacrylate, butyl acrylate, butyl methacrylate, 2-ethylhexyl acrylateand 2-ethylhexyl methacrylate. Further examples of ethylenicallyunsaturated monomers containing no epoxide groups in the molecule areacids, for example acrylic acid and methacrylic acid, acid amides, forexample acrylamide and methacrylamide, aromatic vinyl compounds such asstyrene, α-methylstyrene and vinyltoluene, nitriles such asacrylonitrile and methacrylonitrile, vinyl halides and vinylidenehalides, such as vinyl chloride and vinylidene fluoride, vinyl esters,for example vinyl acetate, and monomers containing hydroxyl groups, forexample hydroxyethyl acrylate and hydroxyethyl methacrylate.

The epoxide group-containing polyacrylate resin usually has an epoxideequivalent weight of from 400 to 2500, preferably from 500 to 1500 andparticularly preferably from 600 to 1200, a number-average molecularweight (determined by gel-permeation chromatism [sic] using apolystyrene standard) of from 1000 to 15,000, preferably from 1200 to7000 and particularly preferably from 1500 to 5000, and a glasstransition temperature (T_(G)) of from 30° to 80° C., preferably from40° to 70° C. and particularly preferably from 40° to 60° C.

The epoxide group-containing polyacrylate resin can be prepared inaccordance with generally well-known methods, by free-radicalpolymerization.

Components (A) containing ethylenically unsaturated double bonds can beprepared by using components which contain ethylenically unsaturateddouble bonds for the preparation of component (A), and/or by reactingfunctional groups, for example hydroxyl groups or epoxide groups, withcompounds which contain ethylenically unsaturated double bonds (e.g.acrylic acid, methacrylic acid and unsaturated fatty acids). Preferredcomponents (A) containing ethylenically unsaturated double bonds areprepared by reacting polyacrylate resins containing hydroxyl and/orepoxide groups with acrylic acid, methacrylic acid or with anunsaturated fatty acid.

The powder coatings according to the invention can in principle containas component (B) any compound which contains carboxyl groups and maycontain ethylenically unsaturated double bonds, which can be employed inpowder coatings and has a melting point of from 20° to 100° C.,preferably from 20° to 80° C. and particularly preferably from 20° to60° C., or a mixture of such compounds. Examples of compounds containingcarboxyl groups which can be employed are carboxyl group-containingpolyester resins, polyanhydrides of polycarboxylic acids, and especiallypolyanhydrides of dicarboxylic acids. As component (B) it is preferredto employ a polyester resin containing carboxyl groups, a mixture ofsuch polyester resins, a compound which contains on statistical averagefrom 1.5 to 5.0 and preferably from 2 to 4 carboxyl groups and from 0 to20 and preferably from 2 to 15 acid anhydride groups and from 0 to 10and preferably from 0 to 5 ethylenically unsaturated double bonds permolecule, or a mixture of such compounds. As component (B) it is veryparticularly preferred to employ a mixture of from 95 to 5 percent,preferably from 95 to 20 percent and particularly preferably from 90 to50 percent by weight of a carboxyl group-containing polyester resin andfrom 5 to 95 percent, preferably from 5 to 80 percent and particularlypreferably from 10 to 50 percent by weight of a compound which containson statistical average from 1.5 to 5 and preferably from 2 to 4 carboxylgroups and from 0 to 20 and preferably from 2 to 15 acid anhydridegroups and from 0 to 10 and preferably from 0 to 5 ethylenicallyunsaturated double bonds per molecule, the sum of the proportions byweight in each case being 100 percent by weight.

The polyester resins containing carboxyl groups which can be employed ascomponent (B) can be prepared by well-known methods, by reacting

(b1) a diol which may be ethylenically unsaturated, or a mixture of suchdiols,

(b2) optionally, a compound which may be ethylenically unsaturated andwhich contains per molecule at least three functional groups selectedfrom hydroxyl, primary amino, secondary amino, carboxyl and acidanhydride groups, with one acid anhydride group being regarded asequivalent to two carboxyl groups, or a mixture of such compounds, and

(b3) a dicarboxylic acid which may be ethylenically unsaturated, adicarboxylic acid anhydride which may be ethylenically unsaturated, or amixture of such dicarboxylic acids and/or dicarboxylic acid anhydrides;

where either the carboxyl group-containing component is employed inexcess or, in a first step, a polyester resin containing hydroxyl groupsis prepared which is then further reacted with a carboxylic acidanhydride to give a polyester resin containing carboxyl groups. Thecarboxyl group-containing polyester resins employed as component (B)should have a number-average molecular weight (determined bygel-permeation chromatography using a polystyrene standard) of from 300to 5000, preferably from 500 to 3000 and particularly preferably from500 to 1700, a glass transition temperature of from 15° to 100° C.,preferably from 20° to 80° C. and particularly preferably from 20° to60° C., and an acid number of from 40 to 300 and preferably from 80 to250. Carboxyl group-containing polyester resins which it is particularlypreferred to employ are obtained by reacting components (b1), (b2) and(b3) in a molar ratio of (b1):(b2):(b3)=from 0.0 to 3.0:1.0:1.5 to 9.0,preferably from 0.0 to 2.0:1.0:2.0 to 8.0 and particularly preferablyfrom 0.5 to 2.0:1.0:2.0 to 6.0.

The reaction of (b1), (b2) and (b3) is carried out by the well-knownmethods of polyester resin preparation. The reaction temperatures areusually from 140° to 240° C. and preferably from 160° to 200° C.

As component (b1) a diol which may be ethylenically unsaturated or amixture of such diols is employed. The term diol refers to an organiccompound which contains two hydroxyl groups per molecule. Examples ofdiols which can be employed are ethylene glycol, propylene glycol,neopentyl glycol, 1,6-hexanediol, cyclohexanediol,cyclohexanedimethanol, hydrogenated bisphenol A, and ethylene oxide orpropylene oxide addition products with bisphenol A, hydrogenatedbisphenol A and diethylene glycol.

As component (b1) it is preferred to employ aliphatic or cycloaliphaticdiols having from 2 to 16 and preferably from 2 to 12 carbon atoms inthe molecule, or mixtures of such diols.

The compound employed as component (b2) may be ethylenically unsaturatedand contains per molecule at least three functional groups selected fromhydroxyl, primary amino, secondary amino, carboxyl and acid anhydridegroups, one acid anhydride group being regarded as equivalent to twocarboxyl groups, or a mixture of such compounds.

Examples of compounds which can be employed as component (b2) are thosecontaining per molecule at least three hydroxyl groups. Examples of suchcompounds are trimethylolpropane, pentaerythritol, trimethylolethane andglycerol.

As component (b2) it is also possible to employ compounds which containtwo primary amino and one hydroxyl group per molecule. An example ofsuch a compound is diaminopropanol.

Compounds which can be employed as component (b2) are also thosecontaining per molecule at least three carboxyl groups or at least oneacid anhydride group and one carboxyl group. Examples of such compoundsare trimellitic acid, trimellitic anhydride, pyromellitic acid andpyromellitic anhydride.

As component (b3) a dicarboxylic acid which may be ethylenicallyunsaturated, a dicarboxylic acid anhydride-which may be ethylenicallyunsaturated or a mixture of such dicarboxylic acids and/or dicarboxylicacid anhydrides is employed. Examples of dicarboxylic acids which can beemployed are saturated and ethylenically unsaturated aliphatic orcycloaliphatic dicarboxylic acids, such as adipic acid, sebacic acid,azelaic acid, dodecanedioic acid, maleic acid, fumaric acid, succinicacid, hexahydrophthalic acid and tetrahydrophthalic acid. Examples ofdicarboxylic acid anhydrides which can employed are the anhydrides ofthe stated acids.

As component (b3) it is also possible to employ aromatic dicarboxylicacids and their anhydrides, for example phthalic acid, isophthalic acidand terephthalic acid. The use of aromatic dicarboxylic acids and theiranhydrides is less preferred.

As compounds which contain on statistical average from 1.5 to 5.0 andpreferably from 2.0 to 4.0 carboxyl groups and from 0 to 20 andpreferably from 2 to 15 acid anhydride groups and from 0 to 10 andpreferably from 0 to 5 ethylenically unsaturated double bonds permolecule it is possible to employ polyanhydrides of saturated orethylenically unsaturated polycarboxylic acids, in particulardicarboxylic acids, or polyanhydrides of mixtures of saturated orethylenically unsaturated polycarboxylic acids, in particulardicarboxylic acids. Such polyanhydrides can be prepared by removingwater from the polycarboxylic acid or mixture of polycarboxylic acids,each two carboxyl groups being converted to one anhydride group. Suchpreparation procedures are well known and therefore require no furtherdiscussion. It is also possible to employ polyanhydrides modified with apolyol, as described in EP-A-299 420, and this polyol modification canalso be effected during the extrusion procedure.

Compounds of the type described above which it is preferred to employare linear polyanhydrides of aliphatic or cycloaliphatic dicarboxylicacids having from 3 to 20 and preferably from 6 to 12 carbon atoms inthe molecule, or linear polyanhydrides of mixtures of such dicarboxylicacids. Specific examples of polyanhydrides which it is preferred toemploy are poly(adipic anhydride), poly(azelaic anhydride), poly(sebacicanhydride), poly(dodecanedioic anhydride) andpoly(cyclohexanedicarboxylic anhydride).

As component (C) it is possible in principle to employ any compoundwhich contains ethylenically unsaturated double bonds and can beemployed in powder coatings, or a mixture of such compounds. Examples ofcompounds which can be employed as component (C) are: maleic anhydride,fumaric acid, acrylamide, methacrylamide, methacrylic acid, crotonicacid, methylenebisacrylamide, methylenebismethacrylamide, styrene,methyl methacrylate, methyl acrylate, butyl acrylate, divinylbenzene,hexanediol diacrylate, trimethylolpropane triacrylate anddivinyldioxane. It is also possible to employ as component (C)polyurethane, polyester or polyacrylate resins which contain hydroxylgroups and are modified with acrylic or methacrylic acid. It ispreferred to employ maleic anhydride as component (C).

As component (D) an initiator for free-radical polymerizations or amixture of initiators for free-radical polymerizations is employed.Initiators for free-radical polymerization are compounds which, atelevated temperatures or under the effect of high-energy radiation,especially under the effect of UV radiation, decompose to give freeradicals. These free radicals are able to initiate free-radicalpolymerizations. Examples of initiators which can be employed are azocompounds, for example azoisobutyronitrile, peroxides, for exampledibenzoyl peroxide, dilauroyl peroxide and di-tert-butyl peroxide, andhydroperoxides, for example cumene hydroperoxide. As component (D) it ispreferred to employ initiators which have a melting point of above 30°C. and a half-life at 120° C. of at least 5 and preferably at least 20minutes.

It is essential to the invention for the mixture of components (A), (B),(C) and (D) to contain from 0.1 to 6.0 mol, preferably from 0.2 to 4.0mol and particularly preferably from 0.3 to 2.5 mol of ethylenicallyunsaturated double bonds per 1000 g of mixture, with component (A)containing from 0 to 50 mole-percent, preferably from 0 to 10mole-percent and particularly preferably from 0 to 5 mole-percent,component (B) containing from 0 to 100 mole-percent, preferably from 40to 100 mole-percent and particularly preferably from 60 to 95mole-percent and component (C) containing from 1 to 100 mole-percent,preferably from 1 to 60 mole-percent and particularly preferably from 5to 40 mole-percent of the ethylenically unsaturated double bondscontained in the mixture.

The powder coatings according to the invention can also contain, inaddition to the mixture of components (A), (B), (C) and (D) employed inaccordance with the invention, other components conventional for powdercoatings, for example crosslinking catalysts, pigments, UV stabilizersand leveling assistants.

The powder coatings according to the invention can be prepared bygenerally well-known methods. The preparation is in general carried outby extruding the mixture of components (A), (B), (C), (D), optionallytogether with further conventional additives, and milling the extrudateobtained. It is also possible to add component (D) after the extrusionof components (A), (B) and (C) and to mill the components (A), (B), (C)and (D) in unison.

The powder coatings according to the invention can be used both withoutpigments (i.e. as clearcoats) and with pigments for the coating of anydesired substrates, for example metal, wood, glass or plastic.

The powder coatings according to the invention can also be used to coatcar bodies. They can be employed in particular, possibly as clearcoats,in two-coat finishes of the basecoat/clearcoat type.

The powder coatings according to the invention can be applied using theapplication techniques which are customary for powder coatings. Thecoating films produced using the powder coatings according to theinvention are stoved at from 120° to 220° C., preferably from 140° to200° C. and particularly preferably from 140° to 180° C. The stovingtime is from 5 to 60 and preferably from 5 to 30 minutes. When aninitiator is employed, as component (D), which can be activated with theaid of high-energy radiation, preferably UV radiation, the coating filmmust additionally also be irradiated with the corresponding radiation.The irradiation is carried out preferably after the storing procedure.

The invention is illustrated in more detail in the examples below. Allparts and percentages are by weight, unless expressly stated otherwise.

1. Preparation of Component (A)

Over a period of 4 hours a mixture of 37.06 parts by weight of methylmethacrylate, 14.40 parts by weight of glycidyl methacrylate, 9.00 partsby weight of n-butyl acrylate and 6.54 parts by weight of styrene isadded at 120° C. to 30.0 parts by weight of xylene. Commencing with theaddition of the monomer mixture, 3.0 parts of tert-butylper-2-ethylhexanoate (TBPEH; manufacturer: Peroxid Chemie) are addedover a period of 4.5 hours. During the addition of the monomer mixtureand the peroxide the reaction temperature is 140° C. This temperature ismaintained after completing the addition of the peroxide for a furtherhour. The xylene is then removed under reduced pressure, and thesynthetic resin is heated to 180° C. and drained off from the reactionvessel. The resulting polyacrylate resin has an epoxide equivalentweight of 686 g/mol.

2. Preparation of Component (B)

2.1 A mixture of 447 parts by weight of hexane-1,6-diol, 338 parts byweight of trimethylolpropane, 792 parts by weight of hexahydrophthalicanhydride and 504 parts by weight of maleic anhydride is slowly heated,and the water formed is removed from the reaction mixture with the aidof a water separator. As soon as the reaction product has reached anacid number of 160, the batch is cooled and at 100° C. is drained offfrom the reaction vessel. The resulting polyester resin, which containsdouble bonds and carboxyl groups and can be polymerized by afree-radical mechanism, has a glass transition temperature (T_(G)) of20° C.

2.2 A mixture of 722 parts by weight of hexane-1,6-diol, 307 parts byweight of trimethylolpropane and 1240 parts by weight of fumaric acid isslowly heated and the water which forms is removed from the reactionmixture with the aid of a water separator. As soon as the reactionproduct has reached an acid number of 160, the batch is cooled and at100° is drained off from the reaction vessel. The resulting polyesterresin, which contains double bonds and carboxyl groups and can bepolymerized by a free-radical mechanism, has a glass transitiontemperature (T_(G)) of 22° C.

2.3 67.2 parts by weight of dodecanedioic acid are weighed out togetherwith 29.8 parts by weight of acetic anhydride and charged to a reactionvessel having a reflux condenser. The mixture is slowly heated untilreflux sets in and is left for 3 hours under reflux. The acetic acidwhich has formed is then removed by distillation. Subsequently a further3.00 parts by weight of acetic anhydride are added to the reactionproduct, and the mixture is heated for one hour at reflux temperature.Finally, the acetic acid which has formed is removed by distillation andthe reaction product is drained off at 90° C. from the reaction vessel.

3. Preparation of Powder Coatings According to the Invention

3.1 800 parts by weight of component (A) prepared as in section 1. arecomminuted in a precutting mill together with 280 parts by weight ofcomponent (B) prepared as in section 2.1, 31 parts by weight ofcomponent (B) prepared as in section 2.3, 19 parts by weight of maleicanhydride, 5 parts by weight of monobutyltin oxide, 6 parts by weight ofa leveling assistant based on polyacrylate resin (Perenol® F 40,manufacturer: Henkel KgaA), 10 parts by weight of benzoin, 26 parts byweight of a first light stabilizer (Tinuvin® 900, manufacturer: CibaGeigy AG) and 16 parts by weight of a second light stabilizer (Tinuvin®144, manufacturer: Ciba Geigy AG) and subsequently premixed. Theresulting mixture is then extruded in a co-kneader from Buss (type PLK46), the temperature in the center of the extrusion chamber being from90°-100° C. The extrudate is cooled rapidly to room temperature andcomminuted using a precutting mill. It is then admixed with 5.9 parts byweight of dicumyl peroxide (Perkadox BC, manufacturer: Akzo), theresulting mixture is ground in an impact mill (ACM 2L from HosokawaMikroPul) to give a powder having an average particle diameter of from30-40 μm, and is screened through a sieve having a pore size of 125 μm.

3.2 800 parts by weight of component (A) prepared as in section 1. arecomminuted in a precutting mill together with 280 parts by weight ofcomponent (B) prepared as in section 2.1, 31 parts by weight ofcomponent (B) prepared as in section 2.3, 19 parts by weight of maleicanhydride, 5 parts by weight of monobutyltin oxide, 6 parts by weight ofa leveling assistant based on polyacrylate resin (Perenol® F 40,manufacturer: Henkel KgaA), 10 parts by weight of benzoin and 353 partsby weight of titanium dioxide, and subsequently premixed. The resultingmixture is then extruded in a co-kneader from Buss (type PLK 46), thetemperature in the center of the extrusion chamber being from 90°-100°C. The extrudate is cooled rapidly to room temperature and comminutedusing a precutting mill (particle diameter from 1 to 3 mm). It is thenadmixed with 7.6 parts by weight of dicumyl peroxide (Perkadox BC,manufacturer: Akzo), the resulting mixture is ground in an impact mill(ACM 2L from Hosokawa MikroPul) to give a powder having an averageparticle diameter of from 30-40 μm, and is screened through a sievehaving a pore size of 125 μm.

3.3 800 parts by weight of component (A) prepared as in section 1. arecomminuted in a precutting mill together with 295 parts by weight ofcomponent (B) prepared as in section 2.2, 31 parts by weight ofcomponent (B) prepared as in section 2.3, 19 parts by weight of maleicanhydride, 5 parts by weight of monobutyltin oxide, 6 parts by weight ofa leveling assistant based on polyacrylate resin (Perenol® F 40,manufacturer: Henkel KgaA), 10 parts by weight of benzoin and 358 partsby weight of titanium dioxide, and subsequently premixed. The resultingmixture is then extruded in a co-kneader from Buss (type PLK 46), thetemperature in the center of the extrusion chamber being from 90°-100°C. The extrudate is cooled rapidly to room temperature and comminutedusing a precutting mill (particle diameter from 1 to 3 mm). It is thenadmixed with 7.6 parts by weight of dicumyl peroxide (Perkadox BC,manufacturer: Akzo), the resulting mixture is ground in an impact mill(ACM 2L from Hosokawa MikroPul) to give a powder having an averageparticle diameter of from 30-40 μm, and is screened through a sievehaving a pore size of 125 μm.

4. Application of the Powder Coatings According to the Invention

The powder coatings prepared as in sections 3.1, 3.2 and 3.3 are appliedelectrostatically to degreased steel panels. After the application thepanels are stoved for 30 minutes at 180° C. The stoved coating filmshave a film thickness of from 40 to 60 μm. They are notable inparticular for their high flexibility (T-bend: 2 mm) with a high degreeof hardness (pencil hardness: 3H-5H, Buchholz hardness 100-125).

We claim:
 1. A powder coating including a film-forming materialconsisting essentially of:(A) from 19.9 to 90.0 percent by weight of asynthetic resin which contains epoxide groups and which may optionallycontain ethylenically unsaturated double bonds, or of a mixture of suchepoxide group-containing synthetic resins; (B) from 9.9 to 80.0 percentby weight of a compound which contains carboxyl groups and which mayoptionally contain ethylenically unsaturated double bonds, or of amixture of such carboxyl group-containing compounds; (C) from 0 to 20percent by weight of a compound which contains ethylenically unsaturateddouble bonds, or of a mixture of such compounds; and (D) from 0.1 to 3.0percent by weight of an initiator for free-radical polymerizations, orof a mixture of initiators for free-radical polymerizations;wherein thesum of the percentages by weight of (A)+(B)+(C)+(D) is 100% by weight,and further wherein the mixture of components (A), (B), (C), and (D)contains from 0.1 to 6.0 mol of ethylenically unsaturated double bondstotal per 1000 g of mixture.
 2. A powder coating according to claim 1,wherein the film-forming material employed is a mixture of from 39.9 to80.0 percent by weight of component (A), from 19.9 to 60.0 percent byweight of component (B), from 0.5 to 10.0 percent by weight of component(C), and from 0.2 to 2.0 percent by weight of component (D).
 3. A powdercoating according to claim 1, wherein component (A) is selected from thegroup consisting of polyacrylate resins containing epoxide groups andmixtures thereof.
 4. A powder coating according to claim 1, whereincomponent (B) is selected from the group consisting of polyester resinscontaining carboxyl groups and mixtures thereof.
 5. A powder coatingaccording to claim 1, wherein the polyester resin employed as component(B) is obtained by reacting(b1) a diol which may optionally beethylenically unsaturated, or a mixture of such diols, (b2) a compoundwhich may optionally be ethylenically unsaturated and which contains permolecule at least three functional groups selected from the groupconsisting of hydroxyl, primary amino, secondary amino, carboxyl, andacid anhydride groups, and mixtures thereof, with one acid anhydridegroup being regarded as equivalent to two carboxyl groups, and (b3) adicarboxylic acid which may optionally be ethylenically unsaturated, adicarboxylic acid anhydride which may optionally be ethylenicallyunsaturated, or mixtures thereof,wherein the molar ratio of(b1):(b2):(b3) is from 0.0 to 3.0:1.0:1.5 to 9.0, to give a polyesterresin having an acid value of from 40 to 300, or a mixture of suchpolyester resins.
 6. A powder coating according to claim 1, wherein thecompound used as component (B) contains on statistical average from 1.5to 5.0 carboxyl groups and from 0 to 20 acid anhydride groups and from 0to 10 ethylenically unsaturated double bonds per molecule, or a mixtureof such compounds.
 7. A powder coating according to claim 5, whereincomponent (B) comprises a mixture of from 95 to 5 percent by weight ofthe polyester resin and from 5 to 95 percent by weight of a secondcompound containing on statistical average from 1.5 to 5.0 carboxylgroups and from 0 to 20 acid anhydride groups and from 0 to 10ethylenically unsaturated double bonds per molecule, or a mixture ofsuch compounds; the sum of the proportions by weight of the polyesterresin and of the second compound always being 100% by weight.
 8. Apowder coating according to one of claims 1, 6, or 7, wherein maleicanhydride is employed as component (C).
 9. A powder coating according toone of claims 1, 6, or 7, wherein component (D) has a half-life at 120°C. of at least 5 minutes.
 10. A powder coating according to one ofclaims 1, 6, or 7, wherein the mixture of components (A), (B), (C), and(D) contains from 0.2 to 4.0 mol of ethylenically unsaturated doublebonds per 1000 g of mixture.
 11. A powder coating according to claim 1,wherein the film-forming material employed is a mixture of from 49.9 to70.0 percent by weight of component (A), from 29.9 to 50.0 percent byweight of component (B), from 1.0 to 5.0 percent by weight of component(C), and from 0.5 to 1.5 percent by weight of component (D).
 12. Apowder coating according to claim 1, wherein the polyester resinemployed as component (B) is obtained by reacting(b1) a diol which mayoptionally be ethylenically unsaturated, or a mixture of such diols,(b2) a compound which may optionally be ethylenically unsaturated andwhich contains per molecule at least three functional groups selectedfrom the group consisting of hydroxyl, primary amino, secondary amino,carboxyl and acid anhydride groups, and mixtures thereof, with one acidanhydride group being regarded as equivalent to two carboxyl groups, and(b3) a dicarboxylic acid which may optionally be ethylenicallyunsaturated, a dicarboxylic acid anhydride which may optionally beethylenically unsaturated, or mixtures thereof,wherein the molar ratioof (b1):(b2):(b3) is from 0.5 to 2.0:1.0:2.0 to 6.0, to give a polyesterresin having an acid value of from 80 to 250, or a mixture of suchpolyester resins.
 13. A powder coating including a film-forming materialconsisting essentially of:(A) from 19.9 to 90.0 percent by weight of acompound selected from the group consisting of polyacrylate resinscontaining epoxide groups and mixtures thereof; (B) from 9.9 to 80.0percent by weight of a compound selected from the group consisting ofpolyester resins containing carboxyl groups and mixtures thereof; (C)from 0 to 20 percent by weight of maleic anhydride; and (D) from 0.1 to3.0 percent by weight of an initiator for free-radical polymerizations,or of a mixture of initiators for free-radical polymerizations having ahalf-life at 120° C. of at least 5 minutes;wherein the sum of thepercentages by weight of (A)+(B)+(C)+(D) is=100% by weight, and furtherwherein the mixture of components (A), (B), (C), and (D) contains from0.1 to 6.0 mol of ethylenically unsaturated double bonds total per 1000g of mixture.
 14. A powder coating according to claim 13, wherein thepolyester resin employed as component (B) is obtained by reacting(b1) adiol which may optionally be ethylenically unsaturated, or a mixture ofsuch diols, (b2) a compound which may optionally be ethylenicallyunsaturated and which contains per molecule at least three functionalgroups selected from the group consisting of hydroxyl, primary amino,secondary amino, carboxyl, and acid anhydride groups, and mixturesthereof, with one acid anhydride group being regarded as equivalent totwo carboxyl groups, and (b3) a dicarboxylic acid which may optionallybe ethylenically unsaturated, a dicarboxylic acid anhydride which mayoptionally be ethylenically unsaturated, or mixtures thereof.wherein themolar ratio of (b1):(b2):(b3) is from 0.5 to 2.0:1.0:2.0 to 6.0, to givea polyester resin having an acid value of from 80 to 250, or a mixtureof such polyester resins.